1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines, and more particularly to an ignition timing control system for an internal combustion engine equipped with a fuel injection amount control system which calculates a fuel injection amount so as to compensate for an amount of fuel (liquid fuel) adhering to the inner wall surface of the intake pipe of the engine after being injected into the intake pipe.
2. Prior Art
In an internal combustion engine wherein liquid fuel such as gasoline is injected into the intake pipe of the engine, part of the injected fuel adheres to the inner wall surface of the intake pipe so that unfavorably a required amount of fuel is not supplied to the combustion chamber of the engine. A fuel supply amount control method for internal combustion engines, which eliminates the above inconvenience, has been known, for example, from Japanese Patent Publication (Kokoku) No. 3-59255, which estimates an amount of fuel (adherent fuel amount) adhering to the inner wall surface of the intake pipe and a fuel amount carried off (drawn) from the inner surface of the intake pipe into the combustion chamber of the engine due to evaporation of adherent fuel or a drawing force of intake air, to thereby control the fuel injection amount for the engine in a manner compensating for the adherent fuel amount and the carried-off fuel amount.
The above-mentioned proposed method has eliminated a drawback of a conventional fuel injection amount control method which is based on the premise that injected fuel is drawn in a sufficient amount into the combustion chamber of the engine, more specifically, a drawback that an insufficient amount of fuel is supplied to the engine due to a larger amount of fuel adhering to the inner surface of the intake pipe immediately after fuel cut, i.e. interruption of fuel supply to the engine has been completed.
However, according to the proposed fuel supply amount control method, which controls the fuel injection amount to be supplied to the engine in a manner compensating for the adherent fuel amount and the carried-off fuel amount, when the engine is recovered from a fuel-cut state into a fuel supply state (hereinafter referred to as "the recovery from F/C"), the following inconvenience arises:
That is, according to an ordinary fuel supply amount control method which does not take into account the adherent fuel amount and the carried-off fuel amount, at the recovery from F/C, part of the injected fuel adheres to the inner wall surface of the intake pipe before flowing into the combustion chamber of the engine. Therefore, the actual amount of fuel supplied into the combustion chamber of the engine is reduced by an amount corresponding to the amount of fuel adhering to the wall surface of the intake pipe and hence the air-fuel ratio (A/F) of a mixture supplied to the engine deviates toward a lean side from a desired value, which results in a slight lean spike such that the fluctuation of the engine torque at the recovery from F/C becomes reduced to suppress a shock which the engine undergoes.
On the other hand, according to the conventional fuel supply amount control method which controls the fuel injection amount supplied to the engine in a manner compensating for the adherent fuel amount and the carried-off fuel amount, when fuel cut is carried out during operation of the engine, the adherent fuel amount is regarded as "0". Accordingly, at the recovery from F/C, a value of adherent fuel amount x carry-off ratio (ratio of an amount of fuel drawn into the combustion chamber of the engine during the present cycle to an amount of fuel having stayed in the intake pipe up to the immediately preceding cycle) becomes "0", and hence a larger amount of fuel than an amount actually required by the engine is injected, which includes an amount of fuel newly adhering to the wall surface of the intake pipe. As a result, no lean spike occurs, so that the fluctuation of the engine torque is not reduced at the recovery from F/C, and hence the engine undergoes a shock which is not suppressed.